1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to a highly luminous light emitting device such as a light emitting diode.
2. Description of the Prior Art
Light emitting diodes with high external quantum efficiency are energy saving devices and have potential for totally replacing incandescent lamps in applications such as traffic signals and automotive lighting in the near future.
External quantum efficiency of a light emitting diode (LED) is given by the product of internal quantum efficiency and extraction efficiency. The internal quantum efficiency is determined by the material property and quality. Direct bandgap III-V semiconductor light emitting diodes which can be formed with good carrier confinement double heterostructure are normally high in the internal quantum efficiency and, however, quite low in the external quantum efficiency due to absorption and internal reflection. The extraction efficiency is the part of the generated light that can escape from a LED chip into ambient air or encapsulating epoxy. Normally the LED chip has a much higher index of refraction, typically 3.4 compared with 1.0 for air and approximately 1.5 for epoxy. Because a critical angle for internal reflection is given by the Snell""s law, this results in a critical angle of 17 degrees for air and 26 degrees for epoxy. Therefore, only the portion of light that emits at an angle less than the critical angle can escape from the LED chip. For most of the semiconductor LEDs, the critical angles are quite small. Most of the light generated at the P/N junction will be totally reflected back and be finally absorbed inside the LED chips.
Attempts have been made to reduce the total internal reflection and subsequent absorption by separating optically thin double heterostructures from the substrate by epitaxial liftoff technique and mounting the heterostructures on a high reflectivity mirror, and texturing the thin-film surface as described in: (1) xe2x80x9cUltrahigh spontaneous emission quantum efficiency, 99.7% internally and 72% externally, from AlGaAs/GaAs/AlGaAs double heterostructuresxe2x80x9d, I. Schnitzer, E. Yablonovitch, C. Caneau, and T. J. Gmitter, Appl. Phys. Lett. 62(2), Jan. 11, 1993, page 131-133; (2) xe2x80x9c30% External quantum efficiency from surface textured, thin-film light emitting diodesxe2x80x9d I. Schnitzer and E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, Appl. Phys. Lett. 63(16); Oct. 18, 1993, page 2174-2176; and (3) xe2x80x9cLight emitting diodes with 31% external quantum efficiency by outcoupling of lateral waveguide modesxe2x80x9d, R. Windisch, P. Heremans, A. Knobloch, P. Kiesel, G. H. Dohler, B. Dutta and G. Borghs, Appl. Phys. Lett. 74(16), Apr. 19, 1999, page 2256-2258. Although the extraction efficiency is improved, however, the epitaxial lift-off technique is not suitable for low cost mass production due to the difficulties in handling such a thin-film LED structure. The thin-film structure is susceptible to breaking into several small pieces during LED chip fabrication process since the thickness thereof is only a few micrometers.
U.S. Pat. No. 4,038,580 discloses a GaAsP electro-luminescent diode having a luminous surface defined by a thinned portion comprising zones with emitting junctions, the zones occupying only a portion of the luminous surface. Although this LED structure is more robust, however, a high yield and reliability are not easy to achieve in fabricating this GaAsP LED. The reasons are: Firstly, the density of current flowing through the emitting junctions is high and the LED reliability is susceptible to being reduced. A large portion of zones 4 and 5 on the thinned portion is removed so as to avoid absorption of the light generated at the emitting junctions during light passage thereof through zone 5. Therefore, the emitting junctions are quite limited in area and consequently high in current density. And secondly, a sandblasting process applied to roughen an undersurface of the thinned portion will damage the thinned portion since which is only about 15 to 40 xcexcm thick. The sandblasting process would also reduce the LED reliability performances.
The present invention discloses a highly luminous light emitting device which includes a substrate, a light emitting layer on the substrate, a current restriction layer on the light emitting layer, a current spreading layer on the current restriction layer, a dielectric layer on the current spreading layer defining an exposed area, a top ohmic contact metal layer on the exposed area, and a bottom ohmic contact metal layer under the substrate. The current spreading layer has a rough top surface. The current restriction layer includes a conductive layer that allows current to flow through, and an insulating layer around the conductive layer. The insulating layer prohibits the current from flowing through in a path between the top ohmic contact metal layer and the bottom ohmic contact metal layer.
The light being blocked is minimized by leading the current to flow in a path from the top ohmic contact metal layer to the bottom ohmic contact metal layer with the restriction of the current restriction layer. And, chances of the light leaving the light emitting device is further increased due to change of the light propagation paths by the rough top surface, which acts as a light emission surface of the light emitting device, of the current spreading layer.
In an embodiment, the light emitting device further includes an etching stop layer between the light emitting layer and the substrate, a cavity substantially in the center of the substrate, and a lower metal layer. The cavity defines a surface and the lower metal layer is formed on the surface. The cavity formed by partially removal of the substrate reduces the substrate absorption.
In an embodiment, the light emitting device further includes a lower confining layer, an upper confining layer, and an active layer sandwiched in between the lower confining layer and the upper confining layer.
By introductions of the rough top surface of the current spreading layer, partial removal of the substrate, the current restriction layer, and optimal layouts of the top and bottom ohmic contact metal layers and the conductive layer, a highly luminous light emitting device of the present invention is obtained.